This invention relates to a new and efficient means of interconnecting a dual barge tow for use in transportation or dredging, that maintains a substantially parallel orientation of the barge's keels to an axis parallel to the propelling vessel's propeller axis to maximize the vessel's propulsive efficiency. Two barges are interconnected at their bow by a bridge element which maintains the spacing of the barges' longitudinal axes while allowing a limited range of pivotal movement to greatly facilitate the makeup and release of the parallel barge/tug arrangement.
When the dual barge arrangement is employed in combination with a center drag arm dredge, the pivotal interconnection of the barge's bows greatly facilitates dredging operations by simplifying the makeup and release of the barge/dredge module for transportation of loaded barges to a dump site.
Currently, trailing suction hopper dredging is perhaps the most viable and used method in the dredging industry. Advantages of this dredging mode are well known, especially to those who create and enforce environmental controls for dredge-spoil disposal operations. The total operation is commonly known as "dredge and haul". As popular as trailing suction hopper dredges have become, they have a significant disadvantage in that their dredge-spoils retention hoppers are too small. Even the largest and most costly of these dredging ships fills its hoppers with retained dredge-spoils in one hour or less. When the hoppers are full, the dredger must stop dredging operations, leave the dredging site, transport dredge-spoils to the disposal site, upon arrival at the disposal site, dump or pump the dredge-spoils from the retention hoppers in a manner that meets all ecological requirements, and then return to the dredging site to repeat the cycle.
Transport and disposal of hopper-retained dredge-spoil has become the dominant consideration in today's harbor channel maintenance dredging and deepening projects as most involve removal of polluted or otherwise noxious dredge-spoils which must be transported to distant disposal sites and there dumped or pumped into special confinements. The interruption of dredging operations posed by the transport and disposal phases of "dredge and haul" operations has become extremely significant. Additionally, special dredge-spoil confinements, designed to protect the environment, make offloading of dredge-spoils from the dredge ship's retention hoppers difficult.
Depending upon the distance between the dredging site and the dump site, the ratio of transport and disposal time to actual dredging time varies, but the average breakdown suggests the ratio is about nine to one; that is nine hours in the transport and disposal phase for every hour in the dredging phase. Thus an expensive, highly specialized dredging ship with an equally expensive and specialized compliment of officers and crew spend nine of every ten hours doing tasks common to all shipping and not requiring the expensive specialization. This break in operations roughly equates to shutting down a factory for nine out of ten hours, while keeping all power on and all employees at their stations. In one important aspect, however, it is worse. A factory resumes operations at the point where it stopped, but the resumption of dredging will be imperfectly prosecuted after the break in dredging operations. The dredge ship cannot return to the range and station at which the last cut was terminated with precision; the tide and currents will have changed and the entire range of dredging parameters which were established via continual observations, monitoring and control of the variables, such as tide, current, position, has been lost due to the nine hour interruption. This inefficiency, the lost dredging time and the discontinuity of dredging, is an inherent result of existing trailing suction hopper dredges having intregal hoppers.
In spite of these great costs due to intrinsic operational inefficiencies, which in all probability will increase with the passage of time, the need for dredging operations will continue to expand to support the increasing requirements of ship traffic in and adjacent to harbors and inland waterways worldwide. This is particularly so due to the substantial increases in the draft and carrying capacity of merchant vessels over the last several years.
Although a growing awareness concerning marine and shoreside ecological considerations has served to emphasize the economic problems of the present hopper dredge operations, the fact is that a need has long existed for a system which is more efficient than that used at present to excavate and dispose of the dredged material.
It is not necessary that trailing suction hopper dredges have internal dredge-spoils retention hoppers. Rather, they can be a composite system consisting of dredge ship to which an endless series of separable hopper units are sequentially supplied, thereby creating a quick-change composite dredging system with potentially infinite hopper capacity. This system retains the dredge ship at the dredging site on a nearly continuous basis while the hopper units and their motive power, such as a tug, transport and dispose of the dredge-spoils.
U.S. Pat. Nos. 3,820,258; 3,973,512; and 4,023,518 disclose systems comprising a dredging ship and abutted, connected barges that were joined using stern notches in the barges and cushioned bows on the trailing ship or barge. However, that system, employing vessels of a special configuration and requiring complex elements for joining and separating the vessels, has proved too costly in construction to provide a practical alternative to the current methods of transporting the dredged material internal to the dredging ship.
Further, when using notched barges for transporting the dredged material, the problems inherent in tug movement of the barges are encountered if the tug, used to transport the barges to and from the disposal site, is not provided with the bow cushions as disclosed in U.S. Pat. No. 3,973,512. Without bow cushions, the tug may use a method of alongside towing known as tandem towing. In tandem towing, the propelling vessel has one barge on each hip, the total wetted surface of such an arrangement is symmetrical and coincident with the longitudinal axis of the propeller. If a single barge alongside arrangement is employed, the propeller axis is offset to the side of the wetted surface's longitudinal axis. Such an arrangement is said to "carry rudder" because the propeller tends to drive the arrangement in a circle, and the powered vessel, the tug or dredge, must use the rudder to compensate. The use of the rudder to compensate limits the amount of rudder available for steering. This unbalanced situation results in an undesirable loss of maneuverability and is particularly notable when attempting to make up the tandem towing arrangement. After hookup of the first barge, the maneuverability of the propelling tug or dredge is so limited that a second tug is often required to make up the dual barge arrangement.